In the past, fluorescent lamps waste has often been disposed of as special waste. The method approaches currently known for reprocessing fluorescent materials principally describe methods which are aimed at recovering the individual components, in particular the rare earth fluorescent materials.
The methods developed are intended to employ economically acceptable means in order to achieve the required quality, which allows unrestricted reuse of the reprocessed products as fluorescent material.
DE-A 34 10 989 describes a method based on two-stage acidic leaching with subsequent precipitation of rare earths by oxalic acid. In the first stage, the halophosphate is separated from the three-band fluorescent material mixture by leaching with nitric acid. The remaining rare earth fluorescent material mixture is re-treated with a nitric acid at least at 90° C. The rare oxides enter into solution. After the solid-liquid separation, the solid material consisting of the insoluble aluminate fluorescent materials is washed, dried and heat treated. Yttrium and europium are precipitated as a mixed oxalate form the filtrate by adding 10% strength oxalic acid. DD-A 246 551 describes complete dissolving of the fluorescent material component containing rare earths in hydrochloric or nitric acid at 90° C. In order to separate the divalent metals, the rare earths are subsequently precipitated as hydroxides from the solution with ammonia. The hydroxides are redissolved in hydrochloric or nitric acid and then precipitated as oxalates. In its present form, however, this rare earth oxide mixture is not suitable for direct use in fluorescent material production, rather only as an intermediate product for an additional elaborate process of separating rare earth elements.
DE-A 196 17 942 bases the reprocessing on treatment of the fluorescent material waste with dilute hydrochloric acid. The halophosphate fluorescent materials are brought into solution as much as possible with dilute hydrochloric acid with the addition of oxidizing agents. After the solid/liquid separation, the rare earth fluorescent materials remaining in the residue are washed thoroughly with deionized water, separated from the aqueous phase and heat treated at T>1200° C.
DE-A 199 18 793 describes a method for three-band fluorescent materials, in which yttrium-europium oxide is recovered as a single component. Its quality allows unrestricted reuse in fluorescent material production. In the first stage, mercury and halophosphate fluorescent materials are dissolved form the fluorescent material waste by means of nitric acid. In the second stage, the rare earth fluorescent material mixture is treated with carbonate alkali. The yttrium-europium oxide selectively enters into solution, and is precipitated as yttrium-europium carbonate and subsequently heat treated to form the oxide.
JP-A 11071111 discloses a method for the extraction of rare earth compounds. A substance, which contains rare earth metals, is treated mechanochemically for a predetermined time in order to modify the crystal structure. The resulting substance is then leached with an acid, specifically at a low concentration, in order to extract the compound containing rare earths. Here, the rare earths are in particular Y and Sc or an element from the lanthanoid group. The mechanochemical treatment is preferably carried out with a highly energetic tool. The acid at a low concentration is preferably hydrochloric acid and sulfuric acid with a concentration of N≦1. Since the rare earth compound can be extracted from the fluorescent lamp waste under comparatively mild conditions, the working environment can be made relatively safe here. In particular, the fluorescent lamp waste may be classed as a promising future municipal source of rare earths. This provides a possibility for recycling rare raw materials.
These publications usually adopt the approach of recovering fluorescent materials “directly” by selective dissolving. The fluorescent materials are either isolated as solid residues after dissolving or, in the case of yttrium-europium oxide “YOE” i.e. Y2O3:Eu, they are precipitated again after dissolving. In the first method, the very fine glass slivers remaining in the residue and the fact that the residue contains all the insoluble components prevent use as a fluorescent material with sufficient quality. In the case of Y2O3:Eu, the luminous efficiency is likewise reduced by co-precipitation of killer elements (for example calcium, terbium) to such an extent that use in fluorescent lamp manufacture is not possible.
The other publications do in fact aim to recover the rare earths in the form of various compounds. In the second method, however, it is extremely unlikely that terbium will be recovered quantitatively. Although the grinding method employed in the fifth method increases the yield of terbium, the data for dissolved aluminum and magnesium show however that this effect is only to a small extent attributable to the digestion of insoluble aluminates.
Since terbium in particular is nowadays one of the economically most important and most expensive rare earth elements, particular value must be placed specifically on complete extraction of this element.
Although the majority of the europium component is obtained as the readily soluble compound Y2O3:Eu (three-band red fluorescent material), from an economical viewpoint the extraction of europium from insoluble compounds in particular is not to be neglected since europium compounds are currently the second most expensive component for the production of three-band fluorescent materials.